Neutralization of hydrophilic gel fibers of a cellulose derivative



United States Patent 3,419,345 NEUTRALIZATION OF HYDROPHILIC GEL FIBERSOF A CELLULOSE DERIVATIVE Esperanza Guandique Parrish, Wilmington, Del.,assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., acorporation of Delaware N0 Drawing. Continuation-impart of applicationSer. No. 148,066, Nov. 27, 1961. This application May 4, 1966, Ser. No.547,455

8 Claims. (Cl. 8-137.5)

ABSTRACT OF THE DISCLOSURE A method for neutralizing acidic or alkalinestructures of freshly for-med highly swellable hydrophilic gel fibers ofa cellulose derivative produces a saft, pliable highly absorbentproduct. Neutralization is accomplished by intimately contacting thefreshly formed unswollen fiber with an aqueous concentrated bufferedsalt solution, e.g., contacting with an aqueous solution of from 15 to30% by weight of sodium sulfate containing a disodium hydrogen phosphatebuffer.

This is a continuation-in-part of US. Ser. No. 148,066, filed Oct. 27,1961.

This invention relates to process. More specifically, it relates to aprocess for neutralizing a freshly formed, highly swellable, hydrophilicgel fiber of a cellulose derivative.

Highly hydrophi'lic fibers, for example, from cellulose ethers, are oldin the art. In one method of preparation they are made by spinning analkaline xanthate solution into an acid bath to regenerate the ether. Inanother process, a cellulosic fiber such as cotton is etherified bysurface application of alkali and an etherifying agent. Neutralizationof the acid or alkaline structures so formed has presented a problem dueto the tendency of the fibers to swell in the presence of water. Suchfibers and structures containing them have not been satisfactory for useas absorbing materials in contact with the body when an aqueous step hasbeen included in the processing either to make the sheet or to bind itdue to the harsh, parchment-like structure that is obtained.

In accordance with the present invention a highly swellable, hydrophilicgel fiber of a cellulose derivative is neutralized by intimatelycontacting the freshly formed unswollen fiber with an aqueous,concentrated, buffered,

salt solution. The freshly formed fiber can be (1) a structurecompletely regenerated in fibrous form by passing (e.g., spinning) analkaline xanthate into an acid bath to coagulate and regenerate thecellulose ether or (2) an etherified fiber of cellulose, the chemicalconstitution of which has been accomplished by surface treatment with anetherifying agent of a cellulosic fiber without complete destruction ofits original morphology. By freshly formed is meant that the cellulosederivative treated has been neither dried, nor swollen by contact withwater, prior to neutralization in accordance with the present invention.The nature of the salt of the aqueous, concentrated salt solution of theprocess of the present invention may vary widely.

The cation of the salt is preferably multivalent but univalent cationsare suitable. The necessary water solubility and cost restricts thepractical cations to magnesium, ammonium and sodium although otheralkali metals would be suitable.

The anion of the salt is preferably multivalent such as sulphate,citrate, bor-ate and phosphate. With a multivalent cation, univalentanions of the classacetate and chloride can be used. Acetates of alkalimetals may also be used but are not preferred.

The salt solution should be at least 0.7 molar and preferably at leastabout 1.2 molar (e.g., about 15% Na SO and is generally used at 30 to C.or higher to minimize precipitation of the salt. 30% salt is oftensuitable.

If the highly hydrophilic fiber contains ionic substituents that canyield an anionic group on the fiber (e.g., carb0X- ylic or sulfonate),the use of a multivalent cation salt will cause cross-linking which willdecrease the absorbency of the dried product. A monovalent cation shouldbe used in this case.

Buffering action is supplied to the salt solution using a conventionalbuffering salt. Suitable systems will be obvious to those skilled in theart and include the following to mention a few: potassium acidphthalate-dipotassium phthalate (pH 5.0-6.2), sodiumdihydrogenphosphate-disod-ium hydrogen phosphate (pH 5.98.0), boricacid-Borax (pH 6.89.0), disodium hydrogen phosphatecitric acid (pH 5-8),citric acid-sodium citrate, and sodium bicarbonate-Na CO Such systemsare normally made by titrating one of the components with a strong baseor acid to the desired pH level.

The invetnion will be more readily understood by reference to thedrawing which is a schematic illustration of the device employed inExample 1. Parts of the device are particularly identified in theexample.

Testing procedures Samples used for tensile tests, basis Weight andbending length determinations are conditioned at F. (21 C.) and 65%relative humidity for at least 24 hours before testing under theseconditions.

Tensile strengths and elongations are measured on 0.5 x 2 inch samplesat an elongation of 50% per minute on an Instron testing machine. Theresults in pounds/ inch, hereafter designated as lbs./in., are dividedby the basis weight to give a normalized result.

Samples are soaked for 5 minutes in distilled water at 21 C. and thenclamped in the tester and broken in air to determine wet strip-tensilestrength.

Bending length is 0.5 the length of a strip of sample that bends underits own weight to a 45 angle. It is determined on a 1 x 6 inch sample ona Drape-Flex Stiffness Tester (made by Fabric Development Tests,Brooklyn, N.Y.).

Basis weights are expressed in ounces/square yards, hereafter designatedas oz./yd. and are based on the Weight of the water-insoluble fiberspresent unless otherwise stated.

The liquid absorption of all samples is determined by soaking a smallsample in an excess of the liquid at 25 C. unless otherwise designated(1 g. in 3000 g. of water) for about 5 minutes. The sample is removedfrom the liquid and spread out to cover a 5 x 5 cm. area. on bleachedsulfiite blotter paper. The sample is placed between layers of blotterpaper and loaded with a 3 kilogram weight to give a pressure of 120g./cm. Pressure is applied for five minutes after which the sample isremoved and weighed, giving the wet weight. Then the sample is dried toconstant weight using a Noble and Woods sheet dryer at C. Absorbencyequals the Water absorbed (wet weight minus dry weight) divided by thedry weight.

All absorbencies of urine are measured in a salt solution of essentiallythe same composition as human urine [16 g. NaCl, 35 g. urea, 2 g. MgSOand 3 g. Cad-I PO H O per liter of solution in distilled water]. Allwater absorbencies are measured in distilled water unless otherwisestated.

The dispersibility is determined in a 250 ml. filter flask having a sidearm at the bottom of the wall and containing a magnetically rotated bar.The bar is 3.8 cm. long by 8 mm. in diameter, weighs 11.73 grams and isrotated at 485 revolutions per minutes. A 3 x 3-inch sample is folded inhalf and inserted under the surface of the water (at the top side arm).Tap water at about 25 C. is added through the bottom tube at a rate of0.70 liters/ minute for a period of 2 minutes. The effiuent liquid fromthe upper side arm is filtered and the residue dried to constant weightat 100 C. to give the weight of fibers dispersed. The contents of thefilter flask are filtered after the test and dried to yield the weightof undispersed fibers. The per cent dispersibility is equal to 100 timesthe weight of fibers dispersed divided by the total Weight of fibersrecovered. Conventional toilet tissues have a dispersibility of 7%.

The wickability of a sample is determined by fastening the ends of a 2 x5.5-inch strip to a perforated metal plate with rubber bands, restingthe end of the plate under about 0.5 inch of distilled water at about 25C. at an angle of 25 to the level of the water and noting the time inminutes for the water to Wick the sample for a distance of inches orless from the top of the water.

The following examples are cited to illustrate the invention. They arenot intended to limit it in any manner.

Example 1 Percent by wt.

Precipitatable cellulose 31.2 Sodium hydroxide 15.75 Water 53.05

Twenty pounds of the above alkali cellulose are charged to a 12-gallonbaratte. The baratte is rotated and warmed to an internal temperature ofC. at which time 2.13 pounds of carbon disulfide and 2.27 pounds ofacrylonitrile is added over a ten-minute interval. During thesimultaneous xanthation and cyanoethylaticn the temperature of thereaction mass is maintained at about C. The reaction requires about 30minutes and its termination is indicated by a cessation of the evolutionof heat by the reaction mass. The cyanoethyl cellulose xanthate productis then dissolved in a 4% sodium hydroxide solution and cooled to 5 C,

The solution so formed is shear precipitated using the technique of US.Patent No. 3,114,747 and the equipment illustrated schematically in thefigure wherein a single circumferential ring of twelve holes 2 having adiameter of 0.02 inch in a /8 inch tubing 1. An aqueous precipitantcontaining 5% by weight sulfuric acid and 15% by weight sodium sulfateis fed at a temperature of about C. and under a pressure of about 120pounds per square inch gauge. The cellulose ether is fed to the tubeprecipitator at 19 C. under a pressure of about 80 pounds per squareinch gauge. The fibrous product is collected on a wire screen andcompressed with a squeeze roller into a /2 inch thick mat. The particlemats are then stored at room temperature for at least 30 minutes toinsure complete regeneration of the cyanoethyl cellulose and thereaftersimultaneously neutralized and dewatered osmotically by being immersedand stirred in a bath containing 17 percent by weight of sodium sulfateand 2 percent by weight of disodium monohydrogen phosphate, adjusted toa pH between 5.0 and 6.0 After draining to approximately 10 percent byweight of solids, the mass is centrifuged to about 30 percent by weightsolids. The particles, ready for further use, are analyzed for nitrogencontent, to determine their d.s. (degree of substitution) which is foundto be 0.42.

The product obtained in this manner is suitable for use in sheetformation, either alone, or as a binder for staple fibers. It is highlyabsorbent.

Example 2 The etherification technique of Example 1 is followed toprovide a solution of cyanoethyl cellulose ether-cellulose Xanthate indilute alkali. The solution is stored at 0 C. for 16 to 24 hours beforeusing.

The aged solution from above is filtered through a filter press,deaerated, heated to about 20 C. and pumped through candle filters to aspinneret having 2200 orifices of 0.003 inch diameter. The solution isextruded from the spinneret into a 48 inch long aqueous coagulating bathcontaining 9% H 50 9% ZnSO and 17% Na SO at -65 C. The filament bundleis forwarded to a feed roll and a skew idler roll where regeneration ofthe yarn is completed. The regenerated yarn is forwarded to another feedroll-idler roll set where it is sprayed with a solution (35 C.)containing 17% Na SO and 3% Na HPO adjusted to a pH of 8.3 with NaOH toneutralize the acid in the yarn and keep it in a deswollen condition.The neutral yarn is passed through squeeze rolls to remove excesssolution and then wound up on packages at 60 yards per minute.

A sample of the dry yarn contains 30.5% of salts and 19% soluble (at 10C.) material. The dry yarn is soft and can be readily separated into theindividual fibers. It can be cut to staple and processed as aconventional fiber. The yarn has the typical following propertiescalculated on a water-insoluble Weight:

Denier per filament (d.p.f.) 2

Tenacity, dry grams per denier (g.p.d.) 1.7 Elongation at the break,dry, percent 9.7 Tenacity, wet g.p.d. (25 C.) 0.03 Water absorbency g./g. 14.4 Urine absorbency g./g. 11.0 Nitrogen, percent 3.1 Degree ofsubstitution (d.s.) cyanoethyl 0.4 COOH, percent 0.5

D.s. carboxyethyl 0.018

Example 3 Thirty-three grams of rayon tow (1.5 d.p.f.) are steeped in18% aqueous NaOH at 18 and pressed to a wet weight of 92 grams. Thefibers are loosened by hand While protected from atmospheric CO by aplastic bag and wound in a loose spiral on a 4 inch bobbin. The bobbinis rotated in a closed resin kettle at 50 and 18 grams of acrylonitrile(AN) is placed in the bottom of the kettle. The AN vapor is reacted withthe cellulose and the resulting cyanoethyl cellulose is hydrolyzed tocarboxyethyl cellulose by the alkali present. After 2 hours the productis acidified in a 5% sulfuric acid-15% sodium sulfate solution, thenneutralized to the salt form in 3% sodium phosphate-17% sodium sulfatesolution adjusted to a pH of 8.5.

The product has a carboxyl d.s. of 0.23 and a cyanoethyl d.s. of 0.034.The fiber is cut into /z-inch length staple. The moist mass contains35.4 cellulosic materials, 92% of which is insoluble in 1% sodiumsulfate.

Hand sheets are made by dispersing 3.95 grams of the above moist staple(equivalent to 1.288 grams of insoluble fiber) plus 0.43 gram of 0.25inch rayon staple in 6 liters of water containing 1% by weight of sodiumsul-.

Example 4 Surgical cotton is hydroxyethylated by treatment with causticsoda and ethylene oxide in turn to obtain a d.s. of 0.7 with anabsorbency of g./ g. in ice tap water.

The fiber is then neutralized in 3% Na HPO 16% Na SO solution with H 80added to a pH of 7 to 8. Hand sheets are made containing 75% of theabove fiber and of 0.25 inch rayon staple. The sheets are dewatered for10 minutes in an aqueous 20% sodium sulfate solution.

Any method of accomplishing intimate contact between the aqueousbuffered salt solution and the gel fiber is suitable, such as byimmersion, preferably with agitation, or spraying. The period of contactneed not be long for most purposes. Periods as high as 5 minutes orlonger are highly advantageous for highly swollen fibers althoughcontact periods as short as 3 to 10 seconds will often accomplishnoticeable results. Excess salt solution may be removed by mechanicalmeans such as wringing or centrifuging or by application of heat toremove water. Salt deposited upon the fiber upon removal of water may beremoved mechanically such as by shaking, scraping or working the fiber,sheet or fabric of which the fiber constitutes an element. For manyapplications removal of excess salt is not necessary, since presence ofsalt does not appear to influence the softness of the resulting product.

The dry, fibrous products of this invention are valuable for use asabsorbing pads for body fluids in a relatively unbonded form such aspapers, batts, webbs, etc., or they can be bonded into integralstructures which have adequate strength and durability in use and aresuitable for disposal in sewage systems after use.

Many equivalent modifications of the present invention will becomeapparent to those skilled in the art from a reading of the above withouta departure from the inventive concept.

What is claimed is:

1. A process for neutralizing a highly swellable, hydrophilic gel fiberof a cellulose derivative which comprises intimately contacting thefreshly formed unswollen fiber with an aqueous, concentrated, bufferedsalt solution, the said salt solution having a pH in the range of fromabout 5.0 to about 9.0.

2. The process of claim 1 wherein the said derivative is an ether.

3. The process of claim 2 wherein the said ether is a cyanoethyl ether.

4. The process of claim 1 wherein the said aqueous salt solutioncomprises from about 15% to about 30% by Weight of Na SO in water and asa buffer, disodium hydrogen phosphate.

5. The process of claim 1 wherein the said neutralizing is an acidifyingreaction.

6. The process of claim 1 wherein the said neutralizing is an alkalizingreaction.

7. The process of claim 1 wherein the said fiber is in a regeneratedform.

8. The process of claim 1 wherein the said fiber is substantially in theform of its original morphology.

References Cited UNITED STATES PATENTS 2,472,877 6/1949 Allquist 8137.5X 2,479,605 8/ 1949 Denyear 8137.5 2,902,334 9/1959 Milne 2641883,146,116 8/1964 Bates 264-182 3,154,614 10/1964 Otsu et a1. 2641883,194,861 7/1965 Bley 264-188 M. WEINBLATT, Primary Examiner.

US. Cl. X.R.

